1. Field of the Invention
The present invention relates to a nitride group compound semiconductor laser device which emits laser light in a range of ultraviolet to blue, and a production method thereof.
2. Description of the Related Art
In recent years, as a material for a semiconductor laser device which emits laser light in the range of ultraviolet to blue, a nitride group compound semiconductor GaxAlyIn1xe2x88x92(x+y)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61) has been used.
With a nitride group compound semiconductor laser device, some structures and production methods thereof have been proposed. Among them, a structure having a current constriction layer capable of simultaneously conducting current constriction and light confinement is expected as a highly reliable structure capable of reducing a driving current and stabilizing an oscillation transverse mode.
As an example, a semiconductor laser device 300 disclosed in Japanese Laid-Open Publication No. 8-97502 is shown in FIG. 3.
The semiconductor laser device 300 includes a sapphire substrate 301, an n-type GaN buffer layer 302, an n-type Al0.2Ga0.8N first cladding layer 303, an In0.15Ga0.85N active layer 304, a p-type Al0.2Ga0.8N second cladding layer 305, an n-type Si current constriction layer 306 having a stripe-shaped opening 320, a p-type Al0.2Ga0.8N third cladding layer 307, and a p-type GaN contact layer 308. The first cladding layer 303, the active layer 304, the second cladding layer 305, the current constriction layer 306, the third cladding layer 307, and the contact layer 308 are partially removed so as to expose the buffer layer 302. A p-side electrode 309 is formed on the contact layer 308, and an n-side electrode 310 is formed on an exposed portion of the buffer layer 302.
In the above-mentioned conventional nitride group compound semiconductor laser device 300, the current constriction layer 306 made of a light-absorbing material is formed in the vicinity of the active layer 304, whereby stable optical waveguide can be realized. However, the current constriction layer 306 is made of Si, and has a forbidden bandgap sufficiently smaller than that of energy corresponding to light generated by the active layer 304, so that an absorption coefficient becomes large (5xc3x97105 cmxe2x88x921) at the current constriction layer 306, which increases a loss in the waveguide. As a result, a semiconductor laser device which oscillates at an oscillating threshold current of about 100 mA or less, required for ensuring reliability, cannot be obtained.
Furthermore, in the conventional nitride group compound semiconductor laser device 300, when laser light is absorbed by the current constriction layer 306 and heat is generated locally in the vicinity of the stripe-shaped opening 320, the crystal is strained due to a large difference (2xc3x9710xe2x88x926) in the thermal expansion coefficient between Si, which is a material for the current constriction layer 306, and a GaN type material. Thus, a semiconductor laser device may be damaged during operation.
In order to solve the above-mentioned problem, it is currently being studied whether GaInN having a large composition of In can be used for the current constriction layer 306. However, it is difficult to grow GaInN having a large composition of In with satisfactory controllability at an ordinary growth temperature. Furthermore, GaInN having a large composition of In has a large absorption coefficient (1xc3x97105 cmxe2x88x921) with respect to a wavelength of laser light. Therefore, a waveguide loss increases, which results in an oscillating threshold current of more than about 100 mA in the same way as in the case using Si, making reliability of the device unsatisfactory.
As described above, in the conventional example, a highly reliable nitride group compound semiconductor laser device which oscillates at an oscillating threshold current of about 100 mA or less in a stable transverse mode cannot be realized.
The main cause for the above problem is as follows: An absorption coefficient in a wavelength region of blue to ultraviolet is large (1xc3x97105 cmxe2x88x921) in the current constriction layer which also functions so as to form a waveguide by light absorption. Therefore, an absorption loss in the waveguide increases, which results in an oscillating threshold current of more than about 100 mA; thus, the device will be thermally damaged during oscillation.
Furthermore, in the case where a material such as Si whose thermal expansion coefficient is largely different from that of a GaN type material is used for the current constriction layer, the device is damaged by local absorption of laser light. Therefore, a highly reliable nitride group compound semiconductor laser device cannot be obtained.
A nitride group compound semiconductor laser device of the present invention includes: a pair of cladding layers; an active layer interposed between the cladding layers; and a current constriction layer having a stripe-shaped opening which is to be a current passage, provided above the active layer, wherein the current constriction layer is formed of a high resistant layer obtained by crystallizing an amorphous or polycrystalline nitride group compound semiconductor by heating.
In one embodiment of the present invention, the current constriction layer is made of GaxAlyIn1xe2x88x92(x+y)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61) containing impurities in an amount of 1xc3x971020 cmxe2x88x923 or more.
Alternatively, a nitride group compound semiconductor laser device of the present invention includes: a pair of cladding layers; an active layer interposed between the cladding layers; and a current constriction layer excluding a stripe-shaped portion which is to be a current passage, provided above the active layer, wherein the current constriction layer is formed of a high resistant layer obtained by irradiating charged particles to crystal of a nitride group compound semiconductor.
In one embodiment of the present invention, a re-evaporation preventing layer or an etching stop layer is provided between the active layer and the current constriction layer.
In another embodiment of the present invention, a re-evaporation preventing layer or an etching stop layer is provided between the active layer and the current constriction layer.
In another embodiment of the present invention, a contact layer is provided above both the current constriction layer and the stripe-shaped opening or the stripe-shaped portion.
According to another aspect of the present invention, a method for producing the above-mentioned nitride group compound semiconductor laser device includes the steps of: growing the first cladding layer and the active layer; growing the amorphous or polycrystalline nitride group compound semiconductor layer on the active layer; conducting wet etching with respect to the nitride group compound semiconductor layer at a temperature of 80xc2x0 C. or lower to form the stripe-shaped opening; and growing the second cladding layer so as to bury the stripe-shaped opening.
In one embodiment of the present invention, the nitride group compound semiconductor layer is grown at a temperature of less than about 700xc2x0 C.
In another embodiment of the present invention, the nitride group compound semiconductor layer is grown at a temperature in a range of about 400xc2x0 C. to about 600xc2x0 C.
Alternatively, a method for producing the above-mentioned nitride group compound semiconductor laser device includes the steps of: growing the first cladding layer, the active layer, and the second cladding layer; and irradiating charged particles to the second cladding layer except for the stripe-shaped portion to form the irradiated portion as the current constriction layer.
Hereinafter, the function of the present invention will be described.
According to the present invention, the current restriction layer provided above the active layer is formed of a high resistant layer which is obtained by crystallizing an amorphous or polycrystalline nitride group compound semiconductor by heating.
The amorphous or polycrystalline nitride group compound semiconductor can be grown with good controllability at a low temperature (less than about 700xc2x0 C.), and the film thus obtained can be rendered highly resistant by including carbon (C) or silicon (Si) in a high concentration (e.g., about 1xc3x971020 cmxe2x88x923 or more). Thus, the current constriction layer effectively blocks a current.
Furthermore, the current constriction layer can be rendered highly resistant by including impurities to have a thickness of about several 10 nm. Therefore, the surface of the contact layer to be grown thereon can easily be made smooth. In the specification, high resistance refers to a resistivity of about 1xc3x97103 xcexa9xc2x7cm or more.
Furthermore, as described later in Embodiment 1, since an absorption coefficient involved in an impurity level can be utilized, an absorption coefficient with respect to a laser oscillating wavelength can be prescribed to be low (e.g., about 5xc3x97103 cmxe2x88x921) which is smaller than an absorption coefficient (about 5xc3x97105 cmxe2x88x921) utilizing an absorption between bandgaps of an ordinary semiconductor. Thus, light confinement in a transverse direction due to light absorption can be realized without increasing an unnecessary waveguide light absorption, and stable transverse mode characteristics can be obtained. Also, the oscillating threshold current can be reduced.
Furthermore, unlike single crystal having a very strong bond grown at a high temperature exceeding about 700xc2x0 C., an amorphous or polycrystalline nitride group compound semiconductor grown at a low temperature (less than about 700xc2x0 C.) can be easily subjected to wet etching at a temperature suitable for photolithography with an alkaline aqueous solution at about 80xc2x0 C. or lower. The stripe-shaped opening is formed by this etching, and the second cladding layer is grown so as to bury the stripe-shaped opening, whereby a current passage can be formed.
Furthermore, in the amorphous or polycrystalline nitride group compound semiconductor, constituent atoms are re-arranged during a temperature increasing process (re-growth after etching), and the semiconductor is formed into single crystal under the condition that it contains impurities in a high concentration. Therefore, a crystal quality of a semiconductor layer to be grown thereon will not be degraded.
Furthermore, since a nitride group compound semiconductor GaxAlyIn1xe2x88x92(x+y)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61) can be used for the current constriction layer, a difference in a thermal expansion coefficient between the current constriction layer and the current passage portion can be decreased. Thus, unlike a conventional semiconductor laser device using Si, according to the present invention, the current constriction layer will not have its crystal strained even when absorbing laser light, thereby preventing a semiconductor laser device from being damaged during an operation.
In the case where GaxAlyIn1xe2x88x92(x+y)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61) is used for the current constriction layer, it is preferable that impurities are added in an amount of about 1xc3x971020 cmxe2x88x923 or more.
Furthermore, by providing a re-evaporation preventing layer between the active layer and the current constriction layer, the active layer can be prevented from evaporating. By providing an etching stop layer between the active layer and the current constriction layer, etching controllability can be enhanced. The etching stop layer and the re-evaporation preventing layer can be formed as one layer.
In another example of a nitride group compound semiconductor laser device of the present invention, the current constriction layer provided above the active layer is formed of a high resistant layer which is obtained by irradiating charged particles to crystal of a nitride group compound semiconductor.
For example, by irradiating charged particles to a part of the cladding layer disposed above the active layer, the stripe-shaped portion (non-irradiated portion) to be a current passage and the current constriction layer (irradiated portion) can be formed with good controllability. The portion irradiated with charged particles includes a high-density carrier stop to be rendered highly resistant, so that the irradiated portion can effectively block a current.
Furthermore, the current constriction layer can be rendered highly resistant by the irradiation of charged particles, and its surface becomes highly impervious to damage. Therefore, the surface of the contact layer to be grown thereon can easily be made smooth.
Furthermore, as described later in Embodiment 2, since a current constriction layer with an increased absorption coefficient, due to a defect level, can be obtained, and an absorption coefficient with respect to a laser oscillating wavelength can be prescribed to be low (e.g., about 5xc3x97104 cmxe2x88x921), which is smaller than an absorption coefficient (about 5xc3x97105 cmxe2x88x921), utilizing an absorption between bandgaps of an ordinary semiconductor. Thus, light confinement in a transverse direction due to light absorption can be realized without increasing an unnecessary waveguide light absorption, and stable transverse mode characteristics can be obtained. Also, an oscillating threshold current can be reduced.
Furthermore, since a nitride group compound semiconductor GaxAlyIn1xe2x88x92(x+y)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61) can be used for the current constriction layer, a difference in a thermal expansion coefficient between the current constriction layer and the current passage portion can be decreased. Thus, unlike a conventional semiconductor laser device using Si, according to the present invention, the current constriction layer will not have its crystal strained even when absorbing laser light, thereby preventing a semiconductor laser device from being damaged during an operation.
Furthermore, by providing a re-evaporation preventing layer between the active layer and the current constriction layer, the active layer can be prevented from evaporating.
Thus, the invention described herein makes possible the advantages of: (1) providing a highly reliable nitride group compound semiconductor laser device which oscillates at a low threshold current in a stable transverse mode; and (2) providing a method for producing such a device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.